Interleaving is a technique used in modern communications systems to help increase immunity to errors and interference during transmission. Interleaving involves dispersing contiguous bits of data in a data stream across a transmission unit so that the data bits that are adjacent in the data stream are no longer adjacent in the transmission unit. As a result of dispersing the data bits, a randomly occurring error or some other source of interference that may be several bits wide that destroys several bits in a transmission unit will only destroy several bits of data from various positions throughout a data stream and not several contiguous data bits.
In communications systems, it is common to use an error correcting code that is designed to correct random errors. Such an error correcting code is not well suited to correct for the loss of several contiguous bits of data, resulting in an error that is not recoverable and requiring that the transmission unit be retransmitted. However, if the several destroyed bits of data were spread throughout the data stream, it is possible for such error correcting codes to correct the resulting randomized data bit errors. Additionally, as data coding schemes become more complex (required to transmit greater data rates) correcting multiple-bit errors becomes more difficult. As error correcting codes are required to correct more multiple-bit errors, they become more expensive in terms of reduced data-rates because of the redundancy required by the error correcting codes themselves becomes a larger percentage of the transmitted data stream.
Typically, the amount of interleaving is specified during the design stage of a communications system. The specification of the amount of the interleaving is crucial to the proper operation of the communications system. If devices in a communications system do not use a consistent amount of interleaving, then the devices would not be able to communicate with each other. Therefore, in most communications systems, the interleaving rate is specified (and is fixed) and is the same for all devices in the system.
However, communications system designers cannot foresee the future and it is impossible for them to anticipate all future sources of errors and interference. For example, according to the IEEE 802.11a technical standard, which is a multi-carrier communications system, adjacent data bits are separated by three subcarriers after they are interleaved. With each subcarrier being 312.5 KHz wide, errors and other sources of interference with bandwidths up to 600 MHz (approximately) can be tolerated without losing adjacent data bits using the specified interleaving rate.
The advantages of the IEEE 802.11a technical standard have become well known and the application of similar modulation techniques (known as multi-carrier modulation techniques) are under consideration for use in new standards. The IEEE 802.11a technical standard uses Orthogonal Frequency Division Multiplexing (OFDM) while another technical standard with application for high-speed digital data transfer using twisted-pair telephone lines uses Discreet Multitone (DMT). Both OFDM and DMT are multi-carrier modulation techniques. While the multi-carrier modulation techniques used in different standard may have differences with one another, the basic idea of using multiple subcarriers to transmit data is a basic foundation of all multi-carrier modulation techniques.
Since the adoption of the IEEE 802.11a technical standard in 1999, a new wireless network standard referred to as the Bluetooth standard has come into widespread use. According to the Bluetooth technical standard, information is transmitted in approximately one-MHz wide frames. Should a one-MHz wide Bluetooth frame collide with a multi-carrier data symbol, such as an IEEE 802.11a data symbol using the interleaving rate specified in the IEEE 802.11a technical standard, multiple adjacent data bits can be destroyed. When such a collision takes place and multiple adjacent data bits are destroyed, often the only remedy is to retransmit the data symbol.
Therefore, a need has arisen for a method that will permit communications systems to adapt to new sources of errors and interference as they develop.